Lubuntu Installer: No mapped devices selectable (cryptsetup)

Hi,

I want to install lubuntu 18.10 on an encrypted disk. I use plain dm-crypt. After finishing the setup that creates my mapper device (and creating a filesystem with mkfs.ext4), I want to start the installation of lubuntu with the 'Lubuntu Installer'. However I am not able to select my mapper devices that I specified earlier in the 'Partitions step'. I have attached some screenshots. What am I missing?